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Unformatted text preview: Biology 171 Friday, October 29, 2010 Lecture 20: Evolutionary Perspective on Life Announcements Next Week in Discussion: Population Genetics/ Evolution Simulations Text Reading Lecture 20: 4 th : Chapter 24 (423-432) 3 rd : Chapter 24 (481-500) Lecture 20: 4 th : Chapter 25 (440-443) 3 rd : Chapter 25 (520-523) Genetic Drift (concluded) Inbreeding Evolution - why the controversy? How science & religion differ and why mixing them is problematic for both. Historical perspectives Darwin & the Theory of Evolution by Natural Selection 1 Genetic Drift - Founder effect A founder event occurs when a group leaves a population, emigrates to a new area, and starts a new population. If the founding group is small, its allele frequencies probably differ from those of the source population. This sampling effect on the new population's allele frequencies is called a founder effect . Founder events and founder effects are especially common in the colonization of isolated habitats such as islands, mountains, caves, and ponds. In 1851, 100 house sparrows from England were released in New York. The species spread across all of North America. Genetic Drift - Founder effect A founder event occurs when a group leaves a population, emigrates to a new area, and starts a new population. If the founding group is small, its allele frequencies probably differ from those of the source population. This sampling effect on the new population's allele frequencies is called a founder effect . Founder events and founder effects are especially common in the colonization of isolated habitats such as islands, mountains, caves, and ponds. In 1851, 100 house sparrows from England were released in New York. The species spread across all of North America. 20 Genetic Drift - Founder effect Has also occurred in human history - movement out of Africa by early humans represented a series of founder events. Non-Africans are considerably less genetically diverse than Africans……. 21 Genetic Drift -Bottleneck Effect This refers to a change in genetic diversity caused by a catastrophic plunge in population size. By chance, some alleles may be lost completely and the relative ratios of the remainder may be signiFcantly altered. 22 Bottleneck Effect: Northern Elephant Seal Original Range: San Francisco to Southern Baja California 23 By the 1890’s hunting for blubber had reduced the population to ≈ 20 individuals that survived on Guadalupe, a remote Mexican island 24 The species has been protected for a century and, remarkably, the population has rebounded to a present day estimate of ≈ 100,000, and it is rapidly re-colonizing its former range. However, this species now has low genetic diversity levels and is potentially more vulnerable to new pathogens 25 Inbreeding (mating between relatives) reduces the frequency of heterozygotes and increases the frequency of homozygotes in each generation ( Figure 25.12 )....
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